This invention is related generally to the art of magnetic storage of computer data, and more particularly to controllers used with disk drives.
With the recent explosion in the use of disk drives in personal and business minicomputers and microcomputers, has developed a number of different techniques for recording and retrieving digital data from magnetic media, particularly disks. Various computers use different disk systems. The result is that data recorded on a floppy disk, for example, by a disk drive of one computer cannot be read by the disk drive of another.
The aspects of disk data recording that vary among the commercially available disk drives are many. The number of data circular tracks on a disk varies. The number of read/write heads, the speed at which the disk rotates, and the particular signal format required to operate the head movement stepper motor, are other aspects that vary among disk drives. Whether the sectors in the tracks are maintained in "hard" or "soft" form, and whether holes in the disk are monitored to identify the location of the start of tracks or sectors, and if so the number and placement of the holes, are among other variables that exist. Particularly, there are different techniques for encoding the computer data into magnetic transitions which are recorded on the disks.
A frequency modulation (FM) technique is a widely used data encoding technique wherein a clock signal is periodically recorded along a track of a disk with a data pulse occurring between clock signals when a "1" is to be recorded and no pulse between clock pulses when a "0" is to be recorded. A difficulty was recognized early with this technique since the recording of each clock pulse causes a large number of magnetic transitions to be recorded on the disk for a given amount of data. Since magnetic media places limitations on how densely the transitions can be spaced relative to each other, the amount of data that can be placed on a given size disk is thus limited.
Therefore, a modified frequency modulation (MFM) technique was developed wherein a clock pulse is recorded only if there has not been a data pulse recorded for a certain period of time. The reduction of the number of magnetic transitions has thus allowed increasing the density of data, thereby increasing the amount of data that can be placed on a given size disk.
Other various data encoding techniques exist as well. Another is group coded recording (GCR) wherein each data nibble (four bits) is converted into a five or six bit word that is written onto disk without any clock signals. The recorded words are chosen to have particular combinations of "1's" and "0's" that allow a maximum amount of data to be recorded for a given number of magnetic flux transitions on the disk. When the data is read off the disk, these words are reconverted to the data nibble originally recorded.
Another technique is run length limited (RLL) which is a superset of the GCR technique and used primarily in Winchester type disk drives.
The incompatibility of data encoding techniques thus makes it quite difficult to adapt one disk drive that has been built to operate with one technique to read from or write upon disks with another technique. Controllers have been provided in commercial disk copiers that have duplicate data encoding and decoding circuits that are selected by switches, but this duplicity is inappropriate, because of the complexity and cost, for use in commercial minicomputers and microcomputers.
Therefore, it is a principal object of the present invention to provide a disk controller capable of handling several different data encoding techniques that is simple and inexpensive.